In a system that uses a touch screen that combines user input and display functions, projective capacitive touch screens are increasingly used. These touch screens do not require pressure or even actual contact to detect a finger used to indicate an input at a location on a two dimensional touch screen. The use of projective capacitive touch screens on smartphones and tablet devices has increased markedly in recent years. These touch screen devices can also be used in many other applications.
In a projective capacitive touch screen display, in one known approach, a grid of conductors are formed with rectangular conductors spaced from one another by a dielectric. A capacitance is formed at each grid intersection by the presence of two plates (the conductor material) spaced by a dielectric. In a touch screen display, the conductive network overlies a display device such as an LED/LCD display, and icons or symbols may be displayed to indicate different actions that can be taken in response to a selection by a user, for example. The touch screen therefore needs to be transparent to allow the user to view the underlying display. Indium Tin Oxide (ITO) touch screens are often used because this conductive material provides the needed transparency. Other conductive materials may be used. A protective layer such as a glass or similarly hard and transparent material is provided over the touch screen. When a conductive second element, such as a human finger, approaches the touch screen the capacitance in the area where the touch is occurring changes (a finger or conductive stylus which creates a parallel capacitive element proximate to the node), and this change can be detected by circuitry coupled to the touch screen to indicate a touch at a specific location. Using this information, a processor can then determine what action is to be taken. For a few non-limiting examples, in a smartphone or tablet device, the processor can launch an application, start or stop a process, input a number that has been touched, or change the display to another screen to reflect the user's input.
FIG. 1 depicts in a simple block diagram a device 10 such as a smartphone or tablet computer presented to illustrate an application for a projective capacitive touch screen. In device 10, a body or chassis 11 supports a touch screen display 13. The device 10 is not limited to smartphones and tablet computers; device 10 can be any device where a user interface using touch can be utilized. Examples include, without limiting the scope of the present application, industrial controls, consumer controls such as thermostats, security alarms, door openers, home lighting, sound systems, video systems, medical devices such as monitors and testers, portable devices such as pagers, music players, video players, fitness monitors, timekeeping devices, radios, stereos, televisions, set top boxes, laptop and “convertible” personal computers, desktop computers, workstations, and the like.
FIG. 2 illustrates a portion of a touch screen device 100 in operation. In FIG. 2, finger 110 is shown touching the surface 112 of a touch screen device 100. The protective screen 108, which may be glass, sapphire glass, or another transparent layer such as polycarbonate that provides mechanical and moisture protection, is touched. Conductor 102 forms a first plate of the capacitor Cp, the panel capacitor at that location. Conductor 104 provides a second plate. The insulating layer 106 spaces the two conductor layers and forms the dielectric for the capacitor Cp. As can be seen in FIG. 2, as the finger 110 (which can also be, for example, a conductive stylus or similar tool) makes contact or almost makes contact with the surface 112, the capacitor Cf is placed in parallel with the capacitor Cp and this changes the capacitance at the location where the touch occurs. A touch screen panel can have tens, hundreds or thousands of the capacitors Cp in an array across the panel. The array of nodes is used to provide an accurate location for the touch, so that the correct location is determined for use by the system.
FIG. 3 depicts a prior known solution for receiving the output of a touch screen panel and for providing a digital data output DO for use by a system 300 that includes the touch screen panel 301.
In FIG. 3, a portion of a touch screen panel 301 that senses the capacitance change on sense capacitor C0 has an output labeled ‘e’ that is coupled to an analog data receiver 310. The analog to data receiver circuit 310 can be implemented as one or more monolithic integrated circuits. In a known prior solution, the circuit 310 is coupled to the panel 301 by a ribbon cable or flex cable connection (not shown) which adds resistance to the system 300. If monolithic integrated circuits are not used, off-the-shelf and/or discrete components can be used instead on a board such as a printed circuit board (PCB) or module to form circuit 310.
In the analog data receiver circuit 310, the first stage 303 implements a trans-impedance amplifier (TIA) and the second stage 305 implements, in this illustrative and non-limiting example, a Sallen-Key band pass filter. The output Vout is an analog voltage that is then converted by an analog to digital converter 311 (ADC) to a digital representation of the input signal ‘e’. The digital data outputs DO are then available for further processing by the system.
In operation, the sense capacitor C0 receives as an input a time varying voltage Vin such as a sinusoidal stimulus signal. Because the current sourced from a capacitor is equal to the capacitance multiplied by the change in voltage, e.g., dQ/dt=I=C dV/dt, the output ‘e’ of the sense capacitor C0 from the panel 301 is taken as a current. The trans-impedance amplifier TIA 303 receives the analog current signal and outputs a corresponding voltage signal to the band pass filter 305, and the band pass filter circuit 305 then outputs the voltage Vout to the ADC 311.
The architecture of the analog data receiver 310 (sometimes referred to as an “analog front end” or AFE) has several disadvantages. To illustrate these, some further analysis of the circuitry is needed.
To obtain the maximum signal to noise ratio (SNR) possible, the input voltage Vin may be driven at a full voltage scale. In order to maintain the same voltage domain at the receiver end, the capacitor Cf1 in circuit 303, the TIA circuit, must be less than or equal to the sense capacitor C0. This arrangement attenuates the input signal, as shown in Equation 1:
                    1        =                              e            Vin                    =                                                    C                ⁢                                                                  ⁢                0                                            Cf                ⁢                                                                  ⁢                1                                      <            1                                              Equation        ⁢                                  ⁢        1            
The noise factor N.F. of the signal passed into the Sallen-Key filter 305 can be written in terms of the Friis equation for multiple stage amplifiers, as shown in Equation 2:
                              N          .          F          .                =                              F            ⁢                                                  ⁢            1                    +                                                    F                ⁢                                                                  ⁢                2                            -              1                                      G              ⁢                                                          ⁢              1                                                          Equation        ⁢                                  ⁢        2            Where F1 is the noise factor of the TIA stage 303, G1 is the gain of the TIA stage 303, and F2 is the noise factor of the second stage 305
The band pass filter nature of the entire system can be expressed as:
                              Vout                      Vin            ⁡                          (                              Tx                ⁡                                  [                  n                  ]                                            )                                      =                                            sC              ⁢                                                          ⁢              0              ⁢              Rf              ⁢                                                          ⁢              1                                      (                                                SCfRf                  ⁢                                                                          ⁢                  1                                +                1                            )                                ⋆                      (                                          Ks                                  C                  ⁢                                                                          ⁢                  1                  ⁢                  r                  ⁢                                                                          ⁢                  1                                                                                                                                                s                        2                                            +                                              s                        (                                                                                                            (                                                              1                                                                  C                                  ⁢                                                                                                                                          ⁢                                  1                                                                                            )                                                        ⁢                                                          (                                                                                                (                                                                      1                                                                          R                                      ⁢                                                                                                                                                          ⁢                                      1                                                                                                        )                                                                +                                                                  (                                                                      1                                                                          R                                      ⁢                                                                                                                                                          ⁢                                      2                                                                                                        )                                                                +                                                                                                      (                                                                          1                                                                              R                                        ⁢                                                                                                                                                                  ⁢                                        3                                                                                                              )                                                                    ⁢                                                                      (                                                                          1                                      -                                      K                                                                        )                                                                                                                              )                                                                                +                                                                                                                                                                                                                                  (                                                      1                                                          C                              ⁢                                                                                                                          ⁢                              2                              ⁢                              R                              ⁢                                                                                                                          ⁢                              2                                                                                )                                                )                                            +                                              (                                                                                                            R                              ⁢                                                                                                                          ⁢                              1                                                        +                                                          R                              ⁢                                                                                                                          ⁢                              3                                                                                                            C                            ⁢                                                                                                                  ⁢                            1                            ⁢                            C                            ⁢                                                                                                                  ⁢                            2                            ⁢                            R                            ⁢                                                                                                                  ⁢                            1                            ⁢                            R                            ⁢                                                                                                                  ⁢                            2                            ⁢                            R                            ⁢                                                                                                                  ⁢                            3                                                                          )                                                                                                                  )                                              Equation        ⁢                                  ⁢        3            
The angular center frequency ω0 that corresponds to the bandwidth being passed by the system is given by Equation 4:
                              ω          ⁢                                          ⁢          0                =                                            (                                                R                  ⁢                                                                          ⁢                  1                                +                                  R                  ⁢                                                                          ⁢                  3                                            )                                      C              ⁢                                                          ⁢              1              ⁢              C              ⁢                                                          ⁢              2              ⁢              R              ⁢                                                          ⁢              3              ⁢              R              ⁢                                                          ⁢              1              ⁢              R              ⁢                                                          ⁢              2                                                          Equation        ⁢                                  ⁢        4            
The bandwidth for the system is given by Equation 5:
                              B          .          W          .                =                                            (                              1                                  C                  ⁢                                                                          ⁢                  1                                            )                        ⁢                          (                                                (                                      1                                          R                      ⁢                                                                                          ⁢                      1                                                        )                                +                                  (                                      1                                          R                      ⁢                                                                                          ⁢                      2                                                        )                                +                                                      (                                          1                                              R                        ⁢                                                                                                  ⁢                        3                                                              )                                    ⁢                                      (                                          1                      -                      K                                        )                                                              )                                +                      (                          1                              C                ⁢                                                                  ⁢                2                ⁢                R                ⁢                                                                  ⁢                2                                      )                                              Equation        ⁢                                  ⁢        5            
The Gain for the system is given by Equation 6:
                    Gain        =                              KC            ⁢                                                  ⁢            0                                C            ⁢                                                  ⁢            1            ⁢            R            ⁢                                                  ⁢            1            ⁢                          B              .              W              .              Cf                                                          Equation        ⁢                                  ⁢        6            
In considering the performance of the overall system, it can be seen from examining Equations 3 and 5 above that the bandwidth B.W. and the peak frequency ω0 depend on the values of discrete components C1, C2, R2, R1, and R3, whereas the gain depends on K, C0 and inversely depends on C1, R1, B.W. and Cf, as given by Equation 6.
In order to increase the gain of the system, K (the gain of the op amp 309) can be increased as seen in Equation 6. However, if K is increased significantly, the band pass filter may enter an instability region, for example it may begin to oscillate. Therefore K should be limited in order to avoid instability or oscillation. The total harmonic distortion (THD) will also increase as K increases. This limits the ability of the circuit designer to increase the gain.
The analog data receiver topology of the known prior solution analog data receiver 310 that is illustrated in FIG. 3 also requires a high number of passive components, including 6 resistors, 3 capacitors, and 2 active analog amplifiers. Then, to finally obtain a digital output, the prior known solution also requires an additional analog to digital converter (ADC) following the analog amplifiers. This circuit topology makes integration of the analog data receiver 310 in a monolithic integrated circuit difficult. Depending on the semiconductor technology node used to manufacture the integrated circuits, multiple integrated circuits can be required to implement the amplifiers and passive components and to then form the analog to digital converter 311 to complete the analog receiver solution for the touch screen. Further, it may not be possible to form multiple signal channels on a single integrated circuit using these prior known approaches.
The use of the large number of passive components on such an integrated circuit makes process tolerances in semiconductor manufacturing difficult, as each of the passive components has to be formed with a precise value. As is known to those skilled in the art, when a circuit has a low tolerance to process and temperature dependent device variations, semiconductor manufacturing yields are lowered and costs therefore increase for the good devices that are produced. Even after the analog receiver data 310 is provided, additional components that process the digital signals are still needed to complete the overall solution, requiring still further circuitry. The silicon area needed to implement the many passive components in analog data receiver 310 makes further integration with the ADC 311 or with other functions difficult, increasing board area and increasing the total number of integrated circuits needed to complete a system with a touch screen display.
Improvements are therefore needed in the analog data receiver circuitry for touch screen devices, such as for the analog front end of touch screen systems, in order to address the deficiencies and the disadvantages of the prior known approaches. Solutions are needed that reduce the number of passive components, reduce the active analog circuitry, increase tolerances to process and temperature device variation, and which improve the performance and increase the level of integration of the circuits.